Chemical and biological separations are routinely performed in various industrial and academic settings to determine the presence and/or quantity of individual species in complex sample mixtures. There exist various techniques for performing such separations.
Mass spectrometry (MS) is an analytical methodology used for quantitative chemical analysis of samples. Molecules in a sample are ionized and separated by a spectrometer based on their respective masses. The separated analyte ions are then detected and a mass spectrum of the sample is produced. The mass spectrum provides information about the masses and in some cases the quantities of the various analyte particles that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on a fragmentation pattern.
Analyte ions for analysis by mass spectrometry may be produced by any of a variety of ionization systems. For example, Atmospheric Pressure Matrix Assisted Laser Desorption Ionization (AP-MALDI), Atmospheric Pressure Photoionization (APPI), Electrospray ionization (ESI), Atmospheric Pressure Chemical Ionization (APCI) and Inductively Coupled Plasma (ICP) systems may be employed to produce ions in a mass spectrometry system. Many of these systems generate ions at or near atmospheric pressure (760 Torr). Once generated, the analyte ions must be introduced or sampled into a mass spectrometer. Typically, the analyzer section of a mass spectrometer is maintained at high vacuum levels from 10−4 Torr to 10−8 Torr. In practice, sampling the ions includes transporting the analyte ions in the form of a narrowly confined ion beam from the ion source to the high vacuum mass spectrometer chamber by way of one or more intermediate vacuum chambers. Each of the intermediate vacuum chambers is maintained at a vacuum level between that of the proceeding and following chambers. Therefore, the ion beam transports the analyte ions transitions in a stepwise manner from the pressure levels associated with ion formation to those of the MASS spectrometer. In most applications, it is desirable to transport ions through each of the various chambers of a mass spectrometer system without significant ion loss. Often an ion guide is used to move ions in a defined direction in the MS system.
Ion guides typically utilize electromagnetic fields to confine the ions radially while allowing or promoting ion transport axially. One type of ion guide generates a multipole field by application of a time-dependent voltage, which is often in the radio frequency (RF) spectrum. These so-called RF multipole ion guides have found a variety of applications in transferring ions between parts of MS systems, as well as components of ion traps. When operated in the presence of a buffer gas, RF guides are capable of reducing the velocity of ions in both axial and radial directions. This reduction in ion velocity in the axial and radial directions is known as “thermalizing” or “cooling” the ions ion populations due to multiple collisions of ions with neutral molecules of the buffer gas. Thermalized beams that are compressed in the radial direction are useful in improving ion transmission through orifices of the MS system and reducing radial velocity spread in time-of-flight (TOF) instruments. RF multipole ion guides create a pseudo potential well, which confines ions inside the ion guide. Typically ion guide operation is limited to pressures below approximately 1 Torr due to problems with ion stagnation inside of the ion guides at higher pressures.
Certain known ion funnel ion optics were developed to overcome the pressure limitations of the certain known ion guides by providing both radial confinements with an RF electrical field and axial acceleration with an electrostatic electrical field. Both the RF and electrostatic fields are generated by an array of concentric rings with progressively reduced ID. Ion funnels can efficiently focus and transfer ions from the entrance to the exit, however neutrals that are embedded into the gas flow also can be transmitted efficiently from the entrance to the exit. Since ion funnels can operate at higher pressures compared to known ion guides, and neutral particle transport is defined by the pressure and flow of the gas inside of the funnel, the problem of separating neutral particles (“neutrals”) from ions become even more actual.
In a known ion funnel, the separation of ions and neutrals is addressed within the ion funnel device by providing an additional central electrode designed to block the path of the neutrals and by supplying an additional voltage to this electrode to divert ions around the central electrode. While this known ion funnel may usefully separate ions and neutrals, the complexity of the additional electrode and additional power supply is not desirable. Moreover, the stability and reliability of such an ion funnel also are problematic due to the contamination on the additional electrode, which results in the charging of the additional electrode and a need to adjust its DC voltage with time.
What is needed, therefore, is a method and apparatus for providing analytes from an ion source to a mass analyzer that overcomes at least the drawbacks of known devices and methods described above.